1. Field of the Invention
The present invention relates generally to optical fiber lasers and amplifiers. The present invention relates more particularly to methods and apparatuses for determining the polarization state and for providing polarization control in optical fiber lasers and amplifiers.
2. Technical Background
Optical fiber lasers and amplifiers are known in the art. In such lasers and amplifiers, rare earth materials disposed in the core of the optical fiber therein absorb pump radiation of a predetermined wavelength, and, in response thereto, provide or amplify light of a different wavelength for propagation in the core. For example, the well-known erbium doped fiber amplifier receives pump radiation having a wavelength of 980 or 1480 nm, and amplifies optical radiation propagating in the core and having a wavelength of about 1550 nm. Lasers and amplifiers generally include one or more amplifier stages, each including a length of fiber that is coupled to one or more pump radiation sources (e.g., pump lasers) and configured to amplify optical radiation passing through its core.
In many cases it can be desirable to characterize the polarization state of the output beam of an amplifier stage, for example, for alignment with other optical elements (either within the laser system or external thereto), or to provide for control of the polarization state of the output beam of the amplifier stage. One conventional technique for characterizing an output beam of an optical fiber amplifier stage is to interrogate the output beam using an optical wedge (or other optical tap) at the optical output of the amplifier stage. The pickup wedge (or other optical tap) will split off a pickup beam from the main output of the optical output of the amplifier stage, which can be used, for example, in polarization monitoring or control. An example of such a system is shown schematically in FIG. 1.
The system of FIG. 1 includes an amplifier system 110, which includes a high power amplifying stage 116. A seed laser 112 is configured to provide input radiation to a polarization controller 113, which is configured to pass polarized radiation to the high power amplifying stage 116. An optical tap 120 (shown here as a free-space optic pickup wedge, but also conventionally provided as a fiber tap) picks out a small fraction of the radiation 150 output from the amplifier system 110 for use in polarization determination and control. In the example of FIG. 1, beam splitter 125 (shown here as a free-space optic beamsplitter, but also conventionally provided as a fiber coupler) splits this tapped radiation and sends it to a polarization determination subsystem 130 and a polarization control system 140. The polarization determination subsystem is configured, for example, to provide an external indication of the polarization state of the output radiation 150, e.g., to a user or an external system. The polarization control system 140 is operatively coupled back to the polarization controller 113, and can be configured to provide feedback control of the polarization state of the output radiation 150.
One important drawback for this type of system is the need to place an optical tap (i.e., the pickup wedge or other optical tap) in the radiation 150 outputted from the amplifier stage. This can provide an additional point of failure of the system, especially when the power outputted by the amplifier stage is high. Moreover, including a pickup wedge or other optical tap in the output radiation can degrade the output beam quality. This can be especially disadvantageous when radiation from multiple amplifier systems needs to be combined, especially when such combination must be performed coherently. An optical element in the output radiation of the amplifier stage can distort the wavefront and thus hinder beam combination efficiency.
Accordingly, there remains a need for improved optical amplifying and laser systems that can provide polarization-controlled output while addressing one or more of these drawbacks.